Polypyrrole

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Electrostatic self-assembly of sandwich-like CoAl-LDH/polypyrrole/ graphene nanocomposites with enhanced capacitive performance Yu Zhang, Dongfeng Du, Xuejin Li, Hongman Sun, Li Li, Peng Bai, Wei Xing, Qingzhong Xue, and Zifeng Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04792 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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ACS Applied Materials & Interfaces

Electrostatic

self-assembly

of

sandwich-like

CoAl-LDH/polypyrrole/graphene nanocomposites with enhanced capacitive performance Yu Zhang,#a Dongfeng Du,#a Xuejin Li,b Hongman Sun,a Li Li,c Peng Bai,b Wei Xing,*ab Qingzhong Xue,a Zifeng Yan*b

a

School of Science, China University of Petroleum, Qingdao 266580, PR China.

b

State Key Laboratory for Heavy Oil Processing, School of Chemical Engineering,

China University of Petroleum, Qingdao 266580, PR China. c

Australian Institute for Bioengineering and Nanotechnology, The University of

Queensland, Brisbane, QLD 4072, Australia.

# Yu Zhang and Dongfeng Du contribute equally to this work.

ACS Paragon Plus Environment


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Abstract A novel

sandwich-like

composite

with

ultrathin

CoAl-LDH

nanoplates

electrostatically assembled on both sides of 2D polypyrrole/graphene (PG) substrate has been successfully fabricated using facile hydrothermal techniques. The PG not only serves as an excellent conductive and structural scaffold to enhance the transmission of electrons and prevents aggregation of CoAl-LDH nanoplates, but also contributes to the enhancement of the specific capacitance. Owing to the homogeneous dispersion of CoAl-LDH nanoplates and its intimate interaction with PG substrate, the resulting CoAl-LDH/PG nanocomposite material exhibits excellent capacitive performance, e.g. enhanced gravimetric specific capacitance (864 F g–1 at 1 A g–1 ), high rate performance (75% retention at 20 A g–1) and excellent cyclic life (almost no degradation in supercapacitor performance after 5000 cycles) in aqueous KOH solution. Furthermore, the assembled asymmetric capacitor is able to deliver superhigh energy density of 46.8 Wh kg–1 at 1.2 kW kg–1 and maintain 90.1% of its initial capacitance after 10000 cycles. These results indicate a rational assembly strategy towards a high-performance pseudocapacitive electrode material with excellent rate performance, high specific capacitance and outstanding cycle stability.

Keywords:

Electrostatic

self-assembly;

LDH;

Polypyrrole;

Supercapacitor;


Graphene;

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Introduction The efficient utilization of renewable energy has become a research highlight in the world due to the fast consumption of fossil fuels by human beings.1-2 Power source technologies provide an option for the conversion and storage of the intermittent renewable energy, with the purpose of reducing reliance on fossil fuels.3 Among these technologies, supercapacitors draw considerable attention because of the highlighted power delivery, high reliability and long cycle.4-5 Recently, layered double hydroxides (LDH), as a pseudocapacitive material, have set off a wave of research owning to its high electrochemical activity, tunable composition and cheap preparation.6 However, the easy aggregation of LDH nanosheets would result in remarkable decrease in surface area and specific capacitance. Additionally, the relatively low mass diffusion within the aggregated LDH nanosheets and high electrical resistance constrain the charge/discharge rate towards future high-power applications. The above drawbacks hinder their further application in the core technologies including modern digital communications and electric vehicles. Tremendous research has been focused on promoting the capacitive characteristics of LDH through incorporation with carbonaceous substrates.7 Construction of LDH/carbon composite structure is believed to remedy the electron transportation resistance of LDH and meanwhile, to enlarge the surface area of LDH

exposed

to

electrolyte. Up to now, various LDH/carbon composites, such as LDH/carbon nanotube,8-9 LDH/graphene,10-11 LDH/carbon fiber,12 have exhibited improved



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capacitive performance. In most cases, however, the performance of LDH in supercapacitors is still restricted by the weak contact between carbonaceous substrates and LDH, resulting in slow kinetics for charge separation. For example, CoAl-LDH/carbon fiber composite manifested a capacitance of 634.3 F g–1 at a low current density of 1 A g–1.13 However, its capacitance retention was 67.3% when the current density increased to 10 A g–1. NiAl-LDH/graphene oxide composite exhibited a capacitance of 915 F g–1 at 1 A g–1.14 Nevertheless, the value decreased to 154 F g–1 at 10 A g–1. CoAl-LDH/graphene foam composite showed a capacitance of 775.6 F g–1 at 0.5 A g–1, which decreased fast to 237 F g–1 at 15 A g–1.15 All the above-mentioned LDH-based composites possess poor rate performance owing to the weak contact between carbonaceous substrates and LDH. Therefore, for the purpose of enhancing the rate capability, it is significant to achieve a composite structure with LDH nanosheets intimately combined with conductive carbon backbones to maximize the utility of LDH nanosheets. Instead of improving the rate property of LDH-based composites, another breakthrough is increasing their energy density by optimizing the carbonaceous components. It is undeniable that conductive carbon backbones, like graphene, can facilitate electron collection and transport.16 Zhang et al. reported that pure CoAl-LDH exhibited a capacitance of 426.6 F g–1 at 1 A g–1, and the capacitance retention at 10 A g–1 was 42%.17 After combining CoAl-LDH with graphene, the capacitance value of CoAl-LDH/graphene composite was 479.2 F g–1 (1 A g–1), but its retention increased to 71% at 10 A g–1. This result indicated that graphene promoted


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the utilization of the transition metal atoms especially at high current densities. However,

many

produced

LDH/graphene

composites

showed

significant

aggregation.17-18 Besides, relatively low specific capacitance of graphene would reduce the total energy density of supercapacitors. An effective strategy to overcome this problem is selecting pseudocapacitive conducting polymer-coated carbon as an alternative substrate. As previously reported, polypyrrole/graphene nanoflakes (PG) possessed high specific capacitance, excellent conductivity and controllable thickness.19 PG can serve as a robust scaffold for the dispersion of CoAl-LDH nanoplates on its surface and kinetically benefit fast electron transport even at high current densities. Thus, it is extremely meaningful to explore a novel electrode material with improved capacitive performance by combining the LDH (high energy density) with PG (superior rate capability). Herein, we for the first time designed and fabricated a sandwich-like architecture with CoAl-LDH

nanoplates electrostatically assembled on both sides of

polypyrrole/graphene substrate (denoted as CoAl-LDH/PG) using a facile hydrothermal route. The PG serves as a conductive and structural support to CoAl-LDH nanoplates. The ultrathin nature of CoAl-LDH nanoplates facilitates the maximized exposure of surface active Co2+/Co3+ to ensure the fast redox reactions at high discharge/charge current densities. More importantly, the face-to-face incorporation largely reduces the contact resistance between PG and CoAl-LDH. Due to this special sandwich-like structure, the CoAl-LDH/PG electrode material possesses increased capacitance with superior rate capability and long-life cycling



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stability.


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Experimental Section Reagents Graphite flakes (99%) was obtained from Qingdao Ruisheng graphite company. The monomer pyrrole (98%) came from Sigma-Aldrich. Chemicals including FeCl3·6H2O, Co(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH and Na2CO3 were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further purification. Preparation of polypyrrole/graphene composite (PG) The aqueous suspension of graphene oxide (GO) was fabricated using modified Hummers approach.20 Firstly, 0.5 g of graphite was added into cold (0 oC) H2SO4 (23 mL) and NaNO3 (0.5 g) solution in a 500 mL flask. 3 g of KMnO4 was slowly put into the flask. The solution was vigorously stirred at 35 oC water bath for 1 h. Then, deionized water (40 mL) was added with the solution temperature raised to about 90 o

C. Deionized water (100 mL) and 30 wt% H2O2 (3 mL) was slowly added into the

solution. Swiftly, the color of the solution was turned into yellow. The obtained solution was ﬁltered and washed with 10% HCl solution and deionized water to remove the metal ions. After centrifugation at 1000 rpm for 10 min, the supernate was centrifuged at 8000 rpm for 15 min. Finally, the sediment was dispersed in deionized water (3.33 mg mL–1) by sonication for 30 min. The PG was fabricated via in-situ polymerization. Typically, 3 mL of GO solution and 0.5 g of pyrrole was mixed in 65 mL of water (0 oC) via 30 min sonication. 25 mL of FeCl3•6H2O aqueous solution was added dropwise with a molar ratio of 2:1 for



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FeCl3•6H2O/pyrrole. After continuous agitation for 8 h, the product was washed thoroughly and dried at 60 oC overnight. Synthesis of CoAl-LDH/PG composite 50 mg of as-prepared PG was dispersed in 50 mL of solution A containing 1.5 mmol Co(NO3)2·6H2O and 0.5 mmol Al(NO3)3·9H2O. 4 mmol of NaOH and 1.5 mmol of Na2CO3 was dispersed in 15 mL deionized water and added dropwise to solution A. After stirring for another 1 h, the as-obtained slurry was transferred to an autoclave. Then, the autoclave was sealed and heated at 120 oC for 16 h. The precipitates were collected, washed and dried at 80 oC overnight. For obtaining CoAl-LDH/PG composites with different CoAl-LDH content, the dosage of PG was altered from 25 to 50, and 100 mg, respectively. The final product was denoted as CoAl-LDH/PG-x, where x represented the theoretical mass ratio between CoAl-LDH and PG. Besides, pure CoAl-LDH was also prepared. Characterizations The morphology was detected by FEI Sirion 200 scanning electron microscopy (SEM). The crystal feature was tested by X'pert PRO MPD X-ray diffraction (XRD) and JEOL-2100UHR transmission electron microscopy (TEM) with selected area electron diffraction (SAED). Fourier transform infrared spectroscopy (FT-IR) was recorded with a CoAllet6700 instrument. X-ray photoelectron spectroscopy (XPS) was measured by PHI 5000 VersaProbe. N2 adsorption/desorption isotherms were collected by a Tristar 3020 analyzer. The Brunauer-Emmett-Teller (BET) surface area was calculated at P/P0 from 0.05 to 0.30. The total pore volume was determined at


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P/P0 of 0.99. NLDFT model using N2 adsorption data was applied to calculate the pore size distribution. Thermogravimetric (TG) curves were obtained on a PerkinElmer Pyris 1 TGA. Zeta-potential analysis was performed on a Brookhaven ZetaPALS. The electrical conductivity was measured by powder resistivity tester (SZT-D, Suzhou Jingge Electronic Co., Ltd). Prior to the measurement, the samples were mixed with conductive black at a mass ratio of 16:3. Electrochemical measurements The as-synthesized active material, polytetrafluoroethylene (PTFE) and acetylene black at a mass ratio of 80:15:5 were mixed in alcohol, and dried at 60 oC overnight. Then, 2 mg of sample was smeared on a nickel foam substrate (1 cm2) as a working electrode. All tests including galvanostatic charge–discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were detected by a CHI 660d electrochemistry workstation in alkaline electrolyte (30 wt% KOH). Platinum and saturated calomel electrode (SCE) were used as the counter and the reference electrode. The asymmetric supercapacitor was also assembled using PG-derived porous carbon (NPCF)19 as the negative electrode and CoAl-LDH/PG-4 as the positive electrode (denoted as CoAl-LDH/PG-4//NPCF). The specific capacitance (F g−1) was computed from the discharge date by the followed formula:

C=

I∆ t m∆V

(1)

where I (A) and ∆t (s) are discharge current and time, ∆V (V) represents potential window, m (g) is the quality of the active material. Energy density (Wh kg−1) and power density (W kg−1) were figured as the followed



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formula:

E=

I Vdt 2mcell ∫

(2)

P = E / ∆t

(3)

where mcell (g) means the mass of electroactive material contained in both electrodes, I (A) represents the corresponding discharge current, V (V) represents the potential window, ∆t (s) is the discharge time.


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Results and Discussion

Scheme 1. Schematic illustration of the fabrication procedure for CoAl-LDH/PG composite. Sandwich-like CoAl-LDH/PG composites were fabricated through a simple hydrothermal synthesis, as shown in Scheme 1. First, the PG precursor was synthesized through in-situ polymerization where polypyrrole was uniformly coated on GO surface. Then, the OH− and CO32+ ions reacted with Co2+ and Al3+ ions to form primary metal hydroxide seeds. During the hydrothermal process, hydroxide seeds gradually grew into nanoplates. The CoAl-LDH nanoplate has a positive zeta potential of 32.1 mV, while the PG has a negative zeta potential of -47.4 mV. Thus, CoAl-LDH/PG composites could be assembled with CoAl-LDH nanoplates dispersed on both sides of PG substrate through the electrostatic interaction. The detailed zeta potential data are shown in Fig. S1. With the increase of the LDH dosage, zeta potential shifts gradually from -8.4 mV (CoAl-LDH:PG=2:1) to 10.3 mV



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(CoAl-LDH:PG=4:1) and then 23.7 mV (CoAl-LDH:PG=8:1), which is due to the gradually enhanced quantity of CoAl-LDH nanoplates adsorbed on PG surface. The morphologies and structures were characterized by the FESEM and TEM. As can be seen, PG (Fig. 1a) shows 2D sheet-like structure and its thickness is about 80 nm. Pure CoAl-LDH (Fig. 1b) is presented as platelets with the diameter of 200-250 nm and thickness of 50 nm. After electrostatic assembly process, the composites (Fig. 1c, S2) show sandwich-like structure with a thin layer of CoAl-LDH nanoplates uniformly coated on PG surface. The obvious lattice fringes (Fig. 1d) indicate the high crystallinity of CoAl-LDH nanoplates. The interplanar spacing of 0.262 nm represents (012) lattice plane of CoAl-LDH. SAED pattern shows that the CoAl-LDH is monocrystalline in structure (the inset of Fig. 1d).21 The uniform combination of CoAl-LDH and PG can be further evidenced by the EDX-elemental mapping analysis. CoAl-LDH/PG-4 (Fig. 1f-j) exhibits the homogeneous distribution of C, N, O, Co and Al elements in the selected area of the sample (Fig. 1e), clearly demonstrating the uniform combination of CoAl-LDH nanoplates and PG substrates. For comparison, SEM morphologies of CoAl-LDH/PG-2 and CoAl-LDH/PG-8 are presented in Fig. S3. Both samples show similar sandwich-like structure of CoAl-LDH/PG-4. CoAl-LDH/PG-2 sample with low CoAl-LDH/PG ratio in Fig. S3a shows sparse dispersion of CoAl-LDH nanoplates on the PG surface, while CoAl-LDH/PG-8 with the highest CoAl-LDH/PG ratio in Fig. S3b shows severe agglomeration and dense packing of LDH nanoplates due to its overloading. The above observations indicate the loading amount of CoAl-LDH is tunable. Obviously,


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the near-monolayer and dense distribution of LDH nanoplates on PG substrate (as in the case of CoAl-LDH/PG-4) can efficiently promote the exposure of active surface of LDH and reduce the contact resistance between LDH and PG, hopefully resulting in improved specific capacitance value and higher rate capability.

Figure 1. SEM images of PG (a), CoAl-LDH (b), and CoAl-LDH/PG-4 (c); (d) HRTEM image of CoAl-LDH/PG-4, the inset is SAED pattern of CoAl-LDH; SEM image of selected area (e) and elemental mappings (f-j) of CoAl-LDH/PG-4. The crystal structures of the samples were investigated by XRD technique. The diffraction peaks (Fig. 2a) could be indexed to those crystal planes, including (003),



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(006), (012), (015), (110) and (113) (JCPDS 51-0045), confirming a successful preparation of the hydrotalcite-like CoAl-LDH phase.22 After combining with PG, the samples show similar diffraction peaks to pristine CoAl-LDH, demonstrating that the addition of PG does not change the crystal structure of CoAl-LDH. It is consistent with the SEM and TEM results. TG/DTA curves (Fig. 2b) were recorded in N2 atmosphere to further clarify the structure of the composite. The TG curve of pure CoAl-LDH displays three stages of mass loss process, including the desorption of the physic- and chemisorbed water from 50 to 200 oC, the subsequent liberation of carbonate ions and dehydroxylation of the hydrotalcite-like layers at 200-330 oC and the further dehydroxylation above 330 o

C.23 The TG curve of the CoAl-LDH/PG-4 shows similar three-step mass losses, also

confirming the successful combination of CoAl-LDH and PG. Moreover, the content of CoAl-LDH nanoplates (Table S1) can be calculated to be 58.4%, 73.8%, and 84.1% for

CoAl-LDH/PG-2,

CoAl-LDH/PG-4,

and

CoAl-LDH/PG-8

composites,

respectively. (The calculating details can be found in the Supporting Information) These values are close to the theoretical mass contents of CoAl-LDH in the composites.


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Figure 2. XRD patterns (a); TG/DTA curves in N2 (b); N2 sorption isotherms (c) and NLDFT pore size distributions (d). To explore the surface area and pore texture, N2 adsorption/desorption measurements were conducted. As shown in Fig. 2c, all the samples exhibit typical IV isotherm, manifesting the formation of mesopores. As illustrated by the NLDFT pore size distributions (Fig. 2d), each sample contains mesopores with size of 3-20 nm. Noticeably, as shown in Table S2, CoAl-LDH/PG-4 has higher surface area and pore volume than pure CoAl-LDH and other composites. However, further increasing the amount of CoAl-LDH will result in its agglomeration and the decrease in surface area of the composite. Because large pore volume and surface area are in favor of electrochemical surface redox reactions and electrolyte penetration, CoAl-LDH/PG-4 electrode is expected to possess better capacitive performance. The presence of functional groups in the prepared samples was investigated by the FT-IR spectroscopy. All samples (Fig. 3a) exhibit a broad peak in 3000-3500 cm−1,



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corresponding to stretching vibrations of N-H in PG, and/or O-H from the OH− in LDH layers and the interlayer water molecules.24 The intense vibration band of 1356 cm−1 confirms the existence of CO32− between the LDH layers.25 And the band at 1075 cm−1 corresponds to typical bending vibrations of M-O-C bond (M means Co or Al).22, 17 In addition, the band at 1622 cm−1 derives from vibration of C-N bond, confirming the introduction of nitrogen species from polypyrrole.26 The chemical composition and valent state of CoAl-LDH/PG-4 were analyzed by the XPS technique. The XPS peaks (Fig. 3b) are attributed to C 1s, O 1s, N 1s, Co 2p and Al 2p, which are well consistent with the EDX results of CoAl-LDH/PG-4 (Table 1). For the C 1s spectrum (Fig. 3c), a pair of peaks at 284.5 and 288.5 eV are corresponding to C-C and O=C-O groups. The O=C-O peak is mainly originated from CO32− between the LDH layers.27 As displayed in the N 1s spectrum (Fig. 3d), a peak centered at 399.7 eV corresponds to the pyrrolic-like nitrogen. The nitrogen atoms (4.9 at.%) come from the polypyrrole in PG, which can also afford faradic capacitance by surface redox reactions.28 The Co 2p spectrum (Fig. 3e) can be fitted with two spin-orbit doublets, corresponding to Co 2p1/2 and Co 2p3/2, and two shake-up satellites. Two major peaks at 797.5 and 781.7 eV indicate that Co2+ coexists with Co3+ in the CoAl-LDH/PG-4 composite.29 In addition, the Co:Al ratio of CoAl-LDH/PG-4 is 2.99 (Table 1), coinciding with feed molar ratio (Co2+/Al3+=3) used in the synthesis.


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Figure 3. FT-IR spectra (a); XPS survey spectrum of the CoAl-LDH/PG-4 composite (b); High-resolution XPS measurements of C 1s (c), N 1s (d) and Co 2p peaks (e). Electrochemical properties The electrochemical performance was evaluated in a three-electrode system. CV curves for CoAl-LDH and its composites (Fig. 4a) show redox peaks at around 0.12 eV and 0.28 eV, clearly revealing their good pseudocapacitive characteristics arising from electrochemically active cobalt species. This is consistent with the reported literature.27 For PG, there are two redox peaks at 0.11 and 0.27 eV, revealing the faradic reaction of pyrrolic-like nitrogen in KOH solution.30 The integrated area for CoAl-LDH/PG-4 electrode is larger than those for the other composite electrodes, and much higher than that for CoAl-LDH, indicating that CoAl-LDH/PG-4 possesses the highest specific capacitance. Moreover, the GCD was measured to study the capacitive property within the potential range of -0.15-0.35 V. GCD curves (Fig. 4b)



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show voltage platforms at 0.15-0.25 V, suggesting typical pseudocapacitive behaviors of these samples as well.8,

27

Additionally, the specific capacitance of

CoAl-LDH/PG-4 is 846.2 F g−1 at 1 A g−1, better than that of CoAl-LDH/PG-2 (524.3 F g−1), CoAl-LDH/PG-8 (698 F g−1), CoAl-LDH (664.2 F g−1) and PG (364 F g−1). These values are in agreement with the CV tests. The high capacitance of CoAl-LDH/PG-4 should be attributed to the near-monolayer CoAl-LDH nanoplates densely distributed on the conductive PG substrate, which ensures the maximum exposure of surface active sites of CoAl-LDH. The significantly increased capacitance of CoAl-LDH/PG-4 also proves that electrochemically anchoring LDH on the surface of conductive PG substrate can be an effective approach to promote the specific capacitance of LDH materials.

Figure 4. (a) Cyclic voltammetric curves of CoAl-LDH/PG composites, CoAl-LDH and PG; (b) Charge-discharge curves of CoAl-LDH/PG composites, CoAl-LDH and PG; XPS spectra of Co 2p for CoAl-LDH/PG-4 after discharge process (c) and charge


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process (d). Moreover, ex-situ XPS was conducted to investigate reversible transformation between Co2+ and Co3+ in the faradic reaction. For the Co 2p spectra after charge and discharge processes (Fig. 4c, d), the Co 2p1/2 and Co 2p3/2 main peaks were separated using Gaussian fitting method. Obviously, two peaks at 780.4 and 795.4 eV are ascribed to Co2+, while the other two peaks at 781.7 and 796.9 eV are assigned to Co3+.31 On the basis of the peak area, the molar ratio of Co3+/Co2+ is 0.87 after discharging process, while it increases to 1.45 after charging process, indicating that the Co2+ turns to Co3+ in charging process. However, the reaction reverses in discharging process. This result confirms that the valence variation of Co atom plays a key role in capacitive energy storage for CoAl-LDH-based materials.

Figure 5. (a) Cyclic voltammetric curves; (b) Specific capacitances with current density; (c) Cycling stability of CoAl-LDH/PG-4 electrode, the inset exhibits the first ten charge-discharge cycles at 5 A g–1; (d) Nyquist plots, the insets are enlarged



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portion near the origin and the equivalent circuit. CV curves of the CoAl-LDH/PG-4 composite with increasing scan rates from 5 to 200 mV s−1 are shown in Fig. 5a. The slight shift of the redox peaks is observed and can be attributed to the mild polarization of the electrode. This mild polarization is ascribed to the low resistance due to the conductive PG substrate and its good contact with CoAl-LDH nanoplates. To prove this, the electrical conductivity of each sample was further tested and listed in Table 2. The conductivity of CoAl-LDH/PG-4 (176.7 S m−1) is about triple of that of the pure CoAl-LDH (62.4 S m−1) and also much higher than that of the CoAl-LDH/PG-8 (89.2 S m−1). This result confirms that it is an effective strategy to reduce the resistance of CoAl-LDH via combining with PG substrate. Furthermore, comparing the specific capacitance at different current densities (Fig. 5b), it shows that 75% of the initial capacitance is still retained for the CoAl-LDH/PG-4 composite at 20 A g−1, which is superior to that of CoAl-LDH (57%) and also better than that of LDH-based materials in previous literatures (Table 3). The excellent rate performance of CoAl-LDH/PG-4 is attributed to its successful construction of fast electron transfer channels, i.e. homogeneous layout of ultrathin CoAl-LDH nanoplates on conductive PG substrate. In this case, conductive PG substrate facilitates charge transfer for efficient redox reactions. Moreover, PG as a steady substrate can avoid the aggregation of the CoAl-LDH nanoplates and expand the LDH/electrolyte interface, which is important to the fast kinetics of this material. To further analyze the cycling stability, the GCD test was repeated at 5 A g–1 for


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5000 cycles (Fig. 5c). CoAl-LDH/PG-4 shows no visible capacitance loss after 5000 charge-discharge cycles. There is also no obvious change of morphology of the composite and aggregation of CoAl-LDH nanoplates (Fig. S4) after the cycling test, confirming its outstanding electrochemical and mechanical stability. EIS was adopted to investigate the charge/ion transfer resistances of the electrode in KOH electrolyte. As shown in the Nyquist plots (Fig. 5d), the intersection of curve at real part represents the equivalent series resistance (Rs), and the diameter of the semicircle corresponds to the charge-transfer resistance (Rct).34 From the inset of Fig. 5d, the CoAl-LDH/PG-4 sample owns lower Rs value and smaller semicircle diameter compared with pure CoAl-LDH and other composites, indicating its lower intrinsic resistance and charge-transfer resistance. The slop of the lines in the transition region corresponds to the Warburg impedance (ZW), which reflects the diffusive resistance of the OH− ion within the electrode material.35 The shorter Warburg region of CoAl-LDH/PG-4 indicates better ion diffusion. Almost vertical line at a low frequency indicates ideal capacitive behavior. In contrast, pure CoAl-LDH and CoAl-LDH/PG-8 present lower slope at the low frequencies, suggesting their inferior capacitive behaviors. To further analyze the impedance spectra, the complex nonlinear least-squares (CNLS) fitting was conducted based on the equivalent circuit (the inset of Fig. 5d).17 The values of RS, CDL, RF, ZW and CL calculated from CNLS method are presented in Table 2. It is obvious that CoAl-LDH/PG-4 possesses relatively lower RS and RF, further confirming its excellent charge transfer and ion transport characteristics. These results provide reliable evidence for the superior rate capability



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of CoAl-LDH/PG-4.

Scheme 2. Illustration of the electronic and ionic transport processes for the CoAl-LDH/PG-4 composite. The improved capacitive performance of the CoAl-LDH/PG-4 can be attributed to four reasons: First, as shown in Scheme 2, conductive PG as structural support for the uniform dispersion of nanometer-sized active materials (CoAl-LDH) can obviously promote whole electrical conductivity of the composite. Second, the unique sandwich-like structure efficiently avoids the aggregation of the CoAl-LDH nanoplates and expose abundant surface active Co2+/Co3+ for rapid faradic reaction as follows:11 Co(OH)2 + OH− ↔ CoOOH + H2O + e−

(4)

Third, the parallel combination largely reduces the contact resistance between CoAl-LDH and PG substrate, which is favorable for fast electron transfer. Forth, nitrogen-doped polymer component can also contribute to the total capacitance by pseudocapacitive mechanism, resulting in large capacitance. Therefore, this sandwich-like CoAl-LDH/PG composite exhibits superior rate performance and enhanced specific capacitance.


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For further demonstrating the potential application of the CoAl-LDH/PG-4 electrode material, the CoAl-LDH/PG-4 composite and the NPCF were assembled into an asymmetric supercapacitor (ASC) in Fig. 6a, which was assigned as CoAl-LDH/PG-4//NPCF. To acquire an appropriate voltage window, CV curves at 100 mV s–1 were tested (Fig. 6b). The result shows that the voltage window can be enlarged to 1.6 V. Further expanding the voltage range (e.g. 0-1.8V) will cause serious electrolysis of water. The shape of CV curves (Fig. 6c) suggests a combination of double-layer capacitance and faradic capacitance. The well retention of CV shape at 200 mV s−1 indicates its swift charge-discharge nature. The GCD curves recorded at various current densities (Fig. 6d) exhibit symmetric triangular shape, indicating the fast I-V response. At the power density of 1.2 kW kg−1, the as-assembled CoAl-LDH/PG-4//NPCF ASC device exhibits a maximum energy density of 46.8 Wh kg−1 in Fig. 6e. Furthermore at a maximum power density of 28.8 kW kg−1 , the ASC device can deliver an energy density of 17.1 Wh kg−1. This result is better than the materials reported before, such as Co3O4//nickel foam/AC (34 Wh kg−1),36 NixCo1-x-LDH-ZTO//AC (23.7 Wh kg−1),37 [email protected]//CBC (36.3 Wh kg−1),38 Co7Ni3-LDH//AC (35.7 Wh kg−1),39 NiCo2O4@MnO2//AC (37.8 Wh kg−1).40 The long-time cycling performance at 10 A g−1 exhibits that the capacitance retention of as-obtained asymmetric supercapacitor is 90.1% after 10000 cycles (Fig. 6f). This result is comparable with or better than other reported ASCs (Table S3), such as Ni(OH)2/graphite//AC (81% after 10000 cycles),32 Ni(OH)2/CNT//AC (83% after 3000 cycles),41 CoMn-LDH//AC (84% after 5000 cycles),42 RGO/CoAl-LDH//AC (90%



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after 6000 cycles).43 The high performance of the CoAl-LDH/PG-4//NPCF asymmetric supercapacitor highlights the promising application of CoAl-LDH/PG composite for high performance ASCs.

Figure 6. (a) Schematic illustration of the CoAl-LDH/PG-4//NPCF asymmetric supercapacitor; (b) Cyclic voltammetric curves at different voltage range; (c) Cyclic voltammetric curves at scan rates of 10-200 mV s−1 and (d) charge-discharge curves at current densities of 0.8-10 A g−1; (e) Ragone plots of the asymmetric supercapacitor and some other devices from previously reported literatures for comparison ( Here, AC, ZTO, CBC, NF are denoted as activated carbon, Zn2SnO4, carbonized bacterial


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cellulose and Ni foam, respectively); (f) The cyclic stability performance, the inset is the initial ten charge-discharge cycles at a current density of 10 A g−1. Conclusion In summary, a novel sandwich-like composite with ultrathin CoAl-LDH nanoplates electrostatically assembled on the surface of 2D PG substrate has been developed by a simple and efficient hydrothermal process. Due to the elaborately-designed fast ion/electron transport channel, expanded CoAl-LDH/electrolyte interface, and high structural stability, the as-obtained CoAl-LDH/PG exhibits high specific capacitance and rate capability. The assembled CoAl-LDH/PG//NPCF asymmetric supercapacitor shows both high energy density and excellent cycling stability. Furthermore, the design strategy can be easily extended to fabricate other high-performance electrode materials for energy-storage application.

Supporting information Zeta potentials of various samples; SEM images of the cross-section of CoAl-LDH/PG-4; SEM images of CoAl-LDH/PG-2 and CoAl-LDH/PG-8; SEM image of CoAl-LDH/PG-4 after 5000 cycles in KOH electrolyte; TG results of various samples and the corresponding mass ratios of CoAl-LDH accompanied by detailed calculations; Surface area and pore volume of the as-prepared samples; Comparison in durability of the CoAl-LDH/PG-4//NPCF ASC device with the ASC devices reported in the references.



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Acknowledgments This work was financially supported by Taishan Scholar Foundation (tsqn20161017, ts20130929), Distinguished Young Scientist Foundation of Shandong Province (JQ201215), National Natural Science Foundation of China (21476264) and Fundamental Research Funds for the Central Universities (15CX05029A, 15CX08009A, 16CX06025A).

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Table 1. The surface chemical compositions from XPS and EDX analysis of PG, CoAl-LDH and CoAl-LDH/PG-4. Samples

Elemental content from XPS analysis

Elemental content from EDX analysis

Co:Al

(at.%)

ratio a

(at.%) Co

Al

C

O

N

Co

Al

C

O

N

PG

N.A.

N.A.

81.62

3.14

15.24

N.A.

N.A.

80.51

2.81

16.68

N.A.

CoAl-LDH

12.58

4.21

37.38

45.83

N.A.

13.66

4.54

36.82

44.98

N.A.

2.98

CoAl-LDH/PG-4

8.15

2.72

39.21

45.0

4.92

8.64

2.93

38.37

44.81

5.25

2.99

a

Co:Al ratio is calculated by the XPS results.

Table 2. Calculated values of RS, CDL, RF, ZW and CL through CNLS fitting of the EIS based on the equivalent circuit in Fig. 5d and electrical conductivity of as-prepared samples. RS (Ω) CoAl-LDH 0.6342 CoAl-LDH/PG-2 0.3381 CoAl-LDH/PG-4 0.3174 CoAl-LDH/PG-8 0.4836 PG 0.3137

CDL (F) 0.0072 0.0216 0.0627 0.0253 0.0159

RF (Ω) 0.7537 0.3447 0.3422 0.5184 0.2573

ZW (F) 4.3241 1.3463 1.5548 3.7328 0.8421

CL (F) 0.1216 0.2536 0.2174 0.2077 0.0814

Conductivity (S m−1) 62.4 199.2 176.7 89.2 463.3

Note: RS is the series resistance of the electrochemical system; CDL is the double-layer capacitance on the grain surface; RF is the charge transfer resistance; Zw is the Warburg impedance and CL is the limit capacitance.


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Table 3. Comparison of the rate performance of LDH-based electrodes in the references. Materials CoAl-LDH-rGO-MnO2 CoAl-LDH/carbon fibers NiAl-LDH/GO CoAl-LDH/graphene CoAl-LDH/RGO CoAl-LDH/Pt NiCoAl-LDH CoAl-LDH/graphene CoAl-LDH@PEDOT Co(OH)2 NiCo-LDH CoAl-LDH/PG-4

Capacitance (F g–1) 718 (1 A g−1) 634 (1 A g−1) 915 (1 A g−1) 600 (1 A g−1) 479 (1 A g−1) 734 (1 A g−1) 1289 (1 A g−1) 712 (1 A g−1) 672 (1 A g−1) 1116 (1 A g−1) 2275 (1 A g−1) 864 (1 A g−1)

Rate capability 54% (10 A g−1) 67% (10 A g−1) 17% (10 A g−1) 57% (10 A g−1) 71% (10 A g−1) 61% (25 A g−1) 57% (30 A g−1) 73% (10 A g−1) 67% (20 A g−1) 38% (10 A g−1) 44% (25 A g−1) 75% (20 A g−1)


Ref. 10 13 14 15 17 21 33 44 45 46 47 This work


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TOC graphic


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Polypyrrole

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